Such a tuning system is already known from the paper "A 10.7 MHz CMOS OTA-R-C bandpass filter with 68 dB dynamic range and on-chip automatic tuning", ISSCC February 1992, 66-67, by M. Steyaert and J. Silva-Martinez. This paper more particularly concerns filters built with operational transconductance amplifiers (OTA's) and associated capacitors C which are commonly known as "OTA-C" filters. The use of this type of filter has proven to be particularly advantageous in the domain of integrated circuit analog filters. One of the advantages of such filters is derived from the controllability of the transconductances of the OTA's incorporated therein. Indeed, by tuning these transconductances the frequency response of the filter can be tuned. This tuning is primarily used to provide a very accurate frequency response in spite of inevitable implementation inaccuracies.
The present invention more particularly relates to a tuning system controlling such an OTA-C filter.
Such tuning systems commonly operate according to the master-slave principle wherein at least one master filter included in a data processing path is tuned by a tuning signal, e.g. a tuning voltage tuning a transconductance as mentioned above, generated by the tuning system which includes a replica of at least a section of this master filter. This replica is called a slave filter and corresponds to the above mentioned tunable filter. The tuning signal has to be so generated that the frequency reponse of the slave filter matches a desired response.
In the aforementioned Steyaent et al document it is proposed to determine this frequency response by tuning separately the quality factor of the total filter and the time constant of the OTA-C integrators included in the filter. The latter tuning which is equivalent to a frequency tuning is more particularly described under the article's sub-heading "Automatic tuning of the frequency" and is the subject of the present invention. For the tuning of the quality factor reference is made to the above-mentioned co-pending patent application of the same inventors as the present one, filed on even date and entitled "Quality factor tuning system and current rectifier used therein" under U.S. Serial Number (Atty. Docket 902-151).
For the frequency tuning it is proposed in the known Steyaent et al document to include a replica of the above mentioned OTA-C integrators of the master filter in the tuning system. The replica of the OTA is then the above mentioned amplifier whereas the replica of C is the mentioned reference capacitor which are used to generate the tuning signal which is also applied to the corresponding integrators in the master filter. The tuning signal is so adjusted that a predetermined ratio exists between the transconductance of the amplifier and the capacitance value of the reference capacitor. In the referenced Steyaent et al document it is further proposed to use a tuning system built according to the well known switched capacitor technique. The latter technique however leads to a rather complicated tuning system as can be verified from the above Steyaent et al paper. Indeed, not only does the proposed tuning system require a relatively large amount of hardware, but the derivation of non-overlapping clocks needed for performing the switched capacitor technique moreover makes it difficult to design this system and increases the hardware cost. Futhermore, the above technique is not suitable when the above-mentioned reference frequency used in the tuning system is relatively high. Indeed, in that case the switching involved adversely affects the performace of the tuning system. Such a high reference frequency may however be necessary to provide a good tuning in case the filter is a high frequency bandpass filter as the tuning will then take into proper account the frequency dependent parasitics inevitably present in such OTA's.